The present invention relates generally to a method and apparatus for detecting the presence of particles on the surface of an object and more particularly to a method and apparatus for detecting contaminant particles on the surface of a patterned semiconductor wafer having repetitive patterns using the principle of light scatttering. An example of a patterned semiconductor wafer having repetitive patterns is a memory wafer.
There are a variety of existing ways for detecting and measuring the number and sizes of particles on the surface of a semiconductor wafer for the purpose of rejecting those wafers which have on their surface one or more particles above certain sizes or an excessive number of particles. One of the more simple methods involves having a human operator inspect the wafer using a light field/dark field microscope. Using the eye, the operator actually counts the number of particles and also identifies the size of the particles, such as those between 1 and 20 microns, and then rejects those wafers which have particles of or above a certain size or which have an excessive number of particles. This method, however, is highly inaccurate and very expensive both in terms of wages for the human operator and in terms of the number of rejects both after the inspection and after production of the chips (when an erroneously passed wafer is found to have an electrical defect, e.g., show circuits, because of the presence of contaminant particles).
In U.S. Pat. No. 5,317,380, issued May 31, 1994, and assigned to Inspex, Inc. there is disclosed a method and apparatus for detecting particles on a surface of an object, such as a virgin or patterned semiconductor wafer, ceramic tile, or the like. In one embodiment, an apparatus is provided in which a scanning beam of laser light is brought to focus as an arcuate scan line on a surface of the object at a grazing angle of incidence using an off-axis hypertelecentric mirror. A pair of light detectors are positioned at a meridional angle of about 30 degrees and at an azimuthal angle of about 4 degrees to measure forward scattered light from the surface. The object is then moved translationally so that the beam can scan another line of the surface. A light trap is provided to trap light that is reflected by the surface, and a series of masks are provided to mask light which is scattered by the hypertelecentric mirror and in the case of pattered objects, light which is diffracted by the pattern imprinted on the object.
In U.S. Pat. No. 4,898,471, issued Feb. 6, 1990, and assigned to Tencor Instruments, a system for detecting particles and other defects on a patterned semiconductor wafer, photomask, or the like is disclosed. The system includes a light source for emitting a beam of light. A polarizing filter is used to polarize the beam of light in a direction substantially parallel to the surface of the patterned semiconductor wafer to be examined. The beam is enlarged in cross-sectional diameter by a beam expander placed along the path of the beam after the polarizing filter. The beam is then caused to scan by a deflection mirror. A telecentric lens brings the scanning beam to focus on the patterned wafer at a shallow angle of incidence, the beam striking the wafer surface substantially parallel to the pattern streets formed on the wafer. A light collection system for detecting side scattered light is positioned in the plane of the scan line. The light collection system, which includes a lens for focusing the side scattered light, a polarizing filter oriented in a direction substantially parallel to the surface of the patterned wafer, and a photomultiplier tube for detecting light incident thereon and transmitting electrical signals in response thereto, receives light scattered in a direction less than 15 degrees above the surface and at angel relative to the beam direction in a range from about 80 degrees to 100 degrees. A processor constructs templates from the electrical signal corresponding to individual patterns and compares the templates to identify particles.
In U.S. Pat. No. 4,806,744 issued Feb. 21, 1989 and assigned to Insystems, Inc., there is disclosed an inspection system which employs a Fourier transform lens and an inverse Fourier transform lens positioned along an optic axis to produce from an illuminated area of a patterned specimen wafer a spatial frequency spectrum whose frequency components can be selectively filtered to produce an image pattern of defects in the illuminated area of the wafer. Depending on the optical components configuration of the inspection system, the filtering can be accomplished by a spatial filter of either the transmissive or reflective type. The lenses collect light diffracted by a wafer die aligned with the optic axis and light diffracted by other wafer dies proximately located to such die. The inspection system is useful for inspecting only dies having many redundant circuit patterns. The filtered image strikes the surface of a two-dimensional photodetector array which detects the presence of light corresponding to defects in only the illuminated on-axis wafer die. Inspection of all possible defects in the portions of the wafer surface having many redundant circuit patterns is accomplished by mounting the wafer onto a two-dimensional translation stage and moving the stage so that the illuminated area continuously scans across the wafer surface from die to die until the desired portions of the wafer surface have been illuminated. The use of a time delay integration technique permits continuous stage movement and inspection of the wafer surface in a raster scan fashion.
In U.S. Pat. No. 4,895,446 to M. C. Maldari et al., there is disclosed a method and apparatus for detecting the presence of particles on the surface of an object such as the front side of a patterned semiconductor wafer. A vertically expanded, horizontally scanning, beam of light is directed onto an area on the surface of the object at grazing angle of incidence. A video camera positioned above the surface detects light scattered from any particles which may be present on the surface, but not specularly reflected light. The surface is angularly repositioned (rotated) relative to the incident light beam so that the diffracted light form the surface and the pattern of lines on the surface is at a minimum. The object is then moved translationally to expose another area to the incident light beam so that the entire surface of the object or selected portions thereof can be examined, one area at a time. The patent also discloses the use of a mark containing a pattern corresponding to the Fourier transform of the patterned surface to mask off light scattered from the pattern on the surface but not any particles that may be present on the surface.
In U.S. Pat. No. 4,377,340 to G. P. Green et al., there is disclosed a method and apparatus for detecting and measuring the number and sizes of impurities on the surface of a material, such as a semiconductor wafer, wherein a beam of high intensity collimated light from a xenon arc lamp is directed onto the surface at normal incidence in the absence of any extraneous light, through a collimating mirror and a pin hole device and where at the particles will scatter the light, and wherein the surface is viewed by a high light sensitive TV camera which is positioned off-axis to pick up scattered light but not specularly reflected light for display on a viewing screen.
In U.S. Pat. No. 4,342,515 to M. Akiba et al., there is disclosed an inspection apparatus for detecting unfavorable foreign matters existent on the surface of an object such as a semiconductor wafer. The apparatus includes a collimated beam generator portion which projects a collimated beam towards the object to be inspected from a side thereof and a mechanism which senses light reflected from the surface of the object, through a polarizer plate. In accordance with the disclosed technique for using the apparatus, the signal-to-noise ratio between a detection signal generated by a pattern of the foreign matter to be detected and a signal generated by a normal pattern of the object surface and sensed as a noise component are said to be enhanced.
In U.S. Pat. No. 3,782,836 to D. F. Fey et al., there is disclosed a surface irregularity analyzing system which includes structure for directing light toward a surface in a direction having a certain angular relationship to the surface. If the light strikes irregularities in the surface it is reflected in a direction having an angular relationship to the surface other than equal and opposite the incident direction. The amount of light reflected from irregularities in the surface is determined, either photographically or photoelectrically using a detector positioned over the surface, to provide an analysis of irregularities in the surface.
In U.S. Pat. No. 2,947,212 to R. C. Woods, there is disclosed a method of detecting surface conditions on a strip of sheet metal having line markings in which light from a light source is directed towards the surface of the sheet metal in a direction generally perpendicular to the line markings. Non-specular reflection in a selected direction which is perpendicular to the lines, and which is preferably between the angle of incidence and the angle of specular reflection, is monitored by a photoelectric cell which is able to detect a surface flaw by variation in the intensity of the reflected light. The light in the incident beam may be polarized and the light in the selected non-specular reflected beam filtered to pass only such polarized light.